Current knowledge on the size distribution of biogenic primary organic aerosols in urban regions with heavy haze pollution is very limited. Here we performed a year-round study focusing on the organic molecular composition of size-segregated aerosol samples collected in urban Beijing during haze and non-haze days to elucidate the seasonal contributions of biomass burning, fungal spores, and plant debris to organic carbon as well as the influences from local emissions and long-range transport.